High frequency electron discharge device



Dec. 20, 1966 c, WARD ETAL 3393,41

HIGH FREQUENCY ELECTRON DISCHARGE DEVICE Filed July 8. 1963 lf Ic l6 v l5 'l I MP 20 2 FIG 2 Eb 22 33 l 9 IO F|G 3 MOST DESIRABLE l -4 OPERATING MA 5| RANGE .3 I Z I 6 -25 I e 5 1 i 5 0 we -2oo -3oo -400 -5oo 3 +5 I I Ec (VOLTS) wrr R T I aoo v r-ls i n ma 'NVENTORS COLLECTING CURTIS E. WARD +25 SECONDARY JAMES RSCHWARTZ ELECTRONS :COLLECTING AT IONS TORNEY United States Patent 3,293,481 HIGH FREQUENCY ELECTRON DISCHARGE DEVICE Curtis E. Ward, Los Altos, and James R. Schwartz, Mountain View, Calif assignors to Varian Associates, Palo Alto, Calif., a corporation of California Fiied July 8, 1963, Ser. No. 293,443 12 Claims. (Cl. 3155.38)

This invention relates in general to high frequency electron discharge devices and more particularly to novel means, which simultaneously vacuum pumps, collects ions and deflects secondary electrons thereby reducing tube noise level, and provides means for monitoring the vacuum levels of such devices, as, for example, a multiple cavity klystron oscillator device.

Present day specifications with regard to high frequency electron discharge devices such as klystron oscillators for use in such diverse applications as pumps for parametric amplifiers and in airborne navigation systems are becoming increasingly demanding with respect to obtaining noise free outputs. Noise generated Within high frequency electron discharge devices is primarily the resultant of undesired spurious variations in the velocity or density of the electron beam. Two primary causal factors in the creation of noise in electron beams are the presence within the beam of ions generated by the collision of both primary and secondary electrons with gaseous particles in the tube and the presence of secondary electrons within the beam caused by primary electrons impinging on such surfaces as the collector of the tube whereupon secondary electrons are generated and directed, if proper precautions are not taken, back into the primary beam itself with a resultant increase in tube noise level.

The resultant problems caused by the presence of ions within the primary beam and methods for eliminating or drawing ions from the electron beam are presented and outlined in US. Patent No. 2,963,605 by R. L. Jepsen et al., assigned to the same assignee as the present invention and in US. Patent No. 2,991,391 by W. L. Beaver also assigned to the same assignee as the present invention. However, such prior art ion draining techniques such as taught by Jepsen et al. and Beaver while adequate do not represent the best solution to the problem in all cases and the present invention is an improvement in the state of the art. For example, the Iepsen et al. device of FIG. 3 utilizes an off axis ion collector 13 in the vicinity of the electron gun region and primary beam which creates electric field lines extending into the main beam and thus, while functioning as an ion drain, also adversely affects the main beam and thus perturbs normal tube electrical parameters existent in the absence of the ion draining electrode. Jepsen et al. in FIG. 4 teaches the symmetrical disposition of an ion draining electrode in the end drift tube which can be biased externally or unbiased externally as shown. This technique is adequate but due to the symmetry of the electrode the resultant electric field lines will tend to refocus any secondary electrons into the primary beam which is undesirable for low noise operation. The location of the ion draining electrodes of FIG. 1 of Jepsen et al. while adequate, is not in the vicinity where maximum secondary electrons are produced and therefore, is inadequate in this respect. Ion draining efiiciency is directly correlated with the location of the ion draining electrode at a point in the tube where maximum secondary electrons exist.

U.S. Patent No. 2,991,391 by W. L. Beaver shows an ion collecting electrode in axial alignment with the central axis of the collector structure. This configuration is symmetric and will produce electric fields which will refocus secondary electrons into the main beam. Furthermore, the location of the electrode also results in primary electrons being intercepted by the ion draining electrode and generation of undesirable secondary electrons from the ion electrode itself.

The present invention obviates all the above mentioned disadvantages of prior art ion draining electrodes and in addition thereto functions as a vacuum pump and vacuum gauge.

Refocusing of secondary electrons into the primary beam is prevented by the present invention by utilization of an asymmetric configuration with respect to the beam axis. Ion draining efiiciency is maximized in the present invention by location of the ion draining electrode in the vicinity of maximum secondary electron production, namely the collector. Primary beam interception is minimized by the off axis location of the ion draining electrode. Vacuum pumping is accomplished in the present invention by positioning an appropriate material on the ion draining electrode and a simple microammeter or the like attached to the external lead of the ion draining electrode permits monitoring of the vacuum conditions within the tube. Normal tube electrical parameters in the absence of an ion draining electrode are not changed by the introduction of an ion draining electrode, as taught by the present invention, in the collector region of the tube Where the primary beam is dissipated and furthermore, the asymmetric ion draining electrode of the present invention sets up an electric field in the collector region which result in the majority of secondary electrons produced by primary beam impingement on the collector being defocused away from the primary beam axis and onto the collector Walls.

Therefore, it is a primary object of the present invention to provide novel means for electron dischange devices which simultaneously vacuum pumps, collects ions and deflects secondary electrons thereby reducing tube noise level, and provides means for monitoring the vacuum levels of such devices.

A feature of the present invention is the provision of an asymmetrical ion draining electrode within the collector region of an electron discharge device.

Another feature of the present invention is the provision of an asymmetrical ion draining electrode within the collector region of an electron discharge device which also simultaneously functions as a continuous vacuum pump for said device.

Another feature of the present invention is the provision of an electron dischange device of the above featured type wherein vacuum monitoring means are operatively connected to said ion draining electrode.

Another feature of the present invention is the provision of an ion draining electrode in an electron discharge device which simultaneously functions to inhibit secondary electrons from entering the primary electron beam in the vicinity of the collector region of said electron dischange device.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a cross-sectional view partly in elevation of a novel two cavity klystron oscillator constructed according to the features of the present invention;

FIG. 2 is a schematic circuit diagram of the klystron of FIG. 1; and

FIG. 3 is a diagram depicting ion collector current (10) v. ion collector voltage (Ec) for the klystron depicted in FIGS. 1 and 2.

Turning to FIG. 1 there is depicted therein a novel electron discharge two cavity klystron oscillator device 1 incorporating an ion draining electrode assembly 2 located in the collector structure 3 of the device. The tube main body 4, cavity resonator side wall, tuning means 5, electron gun assembly 6, including focus electrode 6', cup shaped electron gun support means 7, insulator block 8, cavity partitions or end walls 9, hollow drift tubes 10, and output window assembly 11 are advantageously constructed similarly to the equivalent structure shown and described in co-pending US. patent application Serial No. 85,090, filed January 26, 1961, now U.S. Pat. No. 3,117, 251, and assigned to the same assignee as the present invention. A protective cover member 35 is advantageously positioned over the resonator side wall tuning means 5.

Collector assembly 3 includes a main body 13 preferably of copper having an axial main bore portion 14 and an angularly disposed bore 15 leading to exhaust tubulation 16 having a protective cover 17. I Relatively thick output flange 38 together with the bulk of collector main body portion 13 form a thermal conductive path and thus permit conduction cooling of the klystron.

Disposed in a transverse bore 18 and off-set from the electron beam axis 19 is an ion draining electrode assembly 2, which forms a vacuum tight assembly with the tube interior, said assembly including ion draining electrode means 20 preferably of Kovar having a fitted button end portion or tip 21 of titanium. A steel or the like shell 22 and flanged insert 23 having a ceramic insulating disc 24 preferably of alumina with a Kovar or the like ring 25 brazed thereto surround and support the ion draining electrode means 20.

A copper or the like conductor 26 is conductively connected to electrode means 20 by tab 27. A silastic insulation 28 and a potted silastic end cap 29 cover and protect the ion draining electrode assembly 2. A microammeter 33 is shown connected to conductor 26.

In operation the ion draining electrode assembly 2 functions as follows. As explained previously ion draining electrodes have been previously incorporated in electron discharge devices in attempts to reduce ion oscillations and thus tube noise and to prevent cathode bombardment by ions refocused into the main beam thereby prolonging cathode life. However, the location and configuration of such prior art electrodes was such as to either result in perturbation of the primary electron beam because of the variation of tube electrical parameters and/ or focusing of secondary electrons within the primary beam which perturb the primary beam and increases tube noise and which also cause ion bombardment of the cathode. The ion draining electrode means 20 in the present invention obviates these problems by virtue of being positioned in the collector region of the tube and by virtue of being asymmetrical with respect to the primary beam axis 19. Furthermore, the electrode means 20 is positioned offset from the axis 19 in such a manner that few if any primary electrons diverging in the collector main bore region 14 due to space charge effects will impinge on the electrode.

An extremely simple biasing arrangement as shown in FIG. 2 is employed to obtain any desired ion electrode voltage (BC) with respect to the tube body by simply tapping a portion of supply Eb which determines beam volt age. Ic is equivalent to ion current measured by a monitor such as the microammeter 33. Fluctuations in at a fixed Ec provides an indication of vacuum conditions within the tube. If there is a radical increase in Ic at a fixed Be it is a good indication of a leaky tube.

Since the ion draining electrode means 20 is positioned in the collector main bore 14, the electric field lines 34 which extend into and beyond the beam axis.19 are not detrimental to the useful portion of the main beam and do not require any radical change in focusing techniques to compensate therefor which would be required by an asymmetrically disposed ion draining electrode positioned 1n the cavity regions where useful beam interaction occurs as taught by prior art devices. Furthermore, since the production of undesired ions in electron discharge devices is directly related to the number of collisions between electrons and gas molecules present in the tube and since the collector region where primary beam dissipation occurs is the region where the vast majority of secondary electrons are produced and thus the region where most ions are produced, the present invention, by virtue of positioning the ion draining electrode in the region of primary beam impact, results in increased ion draining efiiciency. Furthermore, the present invention performs the additional function of inhibiting the secondary electrons created by the primary beam impact from entering the useful interaction region of the tube by virtue of the configuration of the electric field lines 34 in the collector region resulting from the positioning of an asymmetrical negatively biased ion draining electrode in the region of the collector main bore 14. Electric field lines 34 will cause a majority of the secondary electrons produced from primary beam impact with the copper collector walls to be deflected as shown by exemplary dashed lines 35 on the side walls of the main bore 14 while simultaneously functioning to attract ions present in the collector region. Ions are attracted to and embedded within the titanium tip at increasing rates as the potential thereon is made increasingly negative as indicated by the la v. Ec characteristics of FIG. 3. Simultaneously these ions will cause sputtering of the titanium tip as described more fully hereinafter. Thus the present invention results in a substantial reduction in density variations of the primary beam due to the introduction therein of spurious secondary electrons and also results in a substantial reduction in ion oscillations caused by the presence of ions in the primary beam and thereby effects in both cases a substantial improvement in tube noise levels.

A tip or end portion 21 made of such reactive materials as titanium, strontium, zirconium or any other transition elements of the 4th, 5th and 6th groups of the periodic table, including the rare earths is bonded by any suitable technique to the ion draining electrode means 20. Positive ions attracted to the tip 21 due to the negative potential impressed thereon will cause disintegration or sputtering of the material of the tip 21 which results in molecules thereof being deposited on the walls of the collector main bore 14. These reactive molecules will function to absorb by entrapment and gettering functions, as they travel to and are deposited on the walls of the collector, free gas molecules present in the tube and thus a continuous pumping process is obtained. Since the walls of the collector are at a positive potential, Eb, positive ions will not cause re-sputtering of the reactive molecules deposited thereon. Thus the ion draining electrode means 20 with the addition of a titanium or the like tip 21 performs the additional function of constantly depositing reactive molecules such as oxides and hybrids, etc. of the tip material on the surface of the collector which reactive molecules absorb, as above indicated, gas molecules and thus continuously evacuate the tube. It is of course to be understood that any materials which are capable of sputtering, gettering and entrapping functions are deemed to be Within the scope of this invention.

FIG. 3 depicts a typical diagram showing Ic v. Ec for the tube of FIG. 1 biased as shown in FIG. 2. It is readily 'aparent that by varying the voltage Be on the ion draining electrode means 20 having a titanium tip 21 that as negative voltages are impressed thereon the electrode collects fewer secondary electrons and begins to function primarily as an ion drain as evidenced by the change in polarity of the Is characteristic. The 10 gradually increases until a point is reached where lesser and lesser numbers of secondary electrons penetrate the electric field lines around the elect-rode 20. This causes a reduction in Ic but not any substantial reduction in the amount of reactive material being deposited on the walls of the collector since the electrons being :far lighter in mass than ions will sputter a proportionately lesser amount of reactive material from the tip 21. However, due to the fact that an impinging secondary electron on tip 21 will in turn produce secondary electrons from the top 21 the Ic characteristic dips gradually as shown in FIG. 3 as larger negative bias voltages are applied to the electrode 20, since less and less secondary electrons are impinging on the tip 21, with the result that drops accordingly since less secondaries are being emitted therefrom. The above explanation is thought to be theoretically correct, however, the exact mechanism may prove to be different upon further study of the pumping, deflect-ing and draining aspects of the present invention. The tact remains, however, that the present invention does result in the simultaneous functions of pumping, ion draining, secondary electron deflection and vacuum monitoring. The region indicated in FIG. 3 as the most desirable operating range is simply a compromise region taking into consideration voltage breakdown problems as higher negative potentials are applied to electrode 20.

It is to be understood that any suitable biasing means may be employed to furnish the bias for electrode 20. Furthermore, it is obvious that the ion drain-ing techniques of the present invention are equally advantageously applied to any electron discharge device such as traveling wave tubes, linear accelerators, etc. In addition, the entire electrode 20 maybe made 013 a reactive material such as used for the tip 21 instead of providing just the end portion of the electrode with a reactive material.

A two cavity klystr-on oscillator such as shown in FIG. 1 was built and tested at 13.3 gc. with an RF. output of 7.7 watts, Eb of 2200 v. -D.C., beam current 112 of around 46 m-a. D.C. heater voltage of 6.3 volts and with an E0 varying from 100 to 2000 V. DC. Audio frequencies in the range of 60-l0,000 c.p.s. at 50 volts R.M.S. were impressed on the ion collector of a two cavity oscillator, constructed according to FIG. 1 and utilizing the electrical parameter specified above while employing a titanium tip on the ion collector electrode, and it was impossible to measure any frequency or amplitude deviations in the carrier, 13.3 gc., which illustrates that normal tube electrical parameters are not change-d by the introduction of the ion draining electrode. Furthermore, a complete absence of any coherent [frequency pips due to ion oscillations was observed in the sideband spectrum of the R.F. output of a two cavity klystron oscillator such as shown in FIG. 1 employing the ion draining electrode assembly 2 operating with DC. potentials, and the random noise level in the sideband spectrum was lower than that in a similar two cavity klystron oscillator without the ion draining electrode. Thus a substantial reduction in both random noise and coherent noise in high frequency electron discharge devices results from the utilization of the present invention.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An electron discharge device comprising an electron gun adapted and arranged to generate and direct an electron beam along a predetermined beam axis, said gun being dispose-d at one end of said axis, a collector structure disposed at the other end of said axis, interaction means disposed along said axis between said gun and collector for interaction with said electron beam, and an ion draining electrode asymmetrically disposed within said collector structure with respect to the beam axis and radially spaced from said beam axis, said ion draining electrode being adapted and arranged to collect ions thereon.

2. An electron discharge device having an electron beam axis including a collector assembly disposed about said axis, said collector assembly comprising an ion draining electrode adapted and arranged to collect ions thereon and asymmetrically disposed in said collector assembly with respect to said axis, said ion draining electrode being radially spaced from said beam axis.

3. A multicavity klyst-ron oscillator device comprising electron gun means adapted and arranged to generate and direct an electron beam along a predetermined axis, said electron gun being disposed at one end of said predetermined axis, a pair of cavity resonators operatively connected to said electron gun means and disposed along said predetermined axis, collector means disposed at the other end of said predetermined axis and operatively connected to said pair or cavity resonators, said collector means having an asymmetrically disposed ion draining electrode therein, wherein said asymmetry is with respect to said predetermined axis, said ion draining electrode being spaced from said predetermined axis.

4. An electron discharge device comprising an electron gun adapted and arranged to generate and direct an electron beam along a predetermined beam axis, said gun being disposed at one end of said axis, a collector structure disposed at the other end of said axis, interaction means disposed along said axis betweensaid gun and collector for providing an interaction means for said electron beam, and an ion draining electrode asymmetrically disposed within said collector stnucture with respect to the beam axis, said ion draining electrode being adapted and arranged to collect positive ions thereon, said ion dr-a-ining electrode having a portion thereof made of a reactive substance selected from the group consisting of reactive materials such as titanium, .strontium, zirconium and any other transition elements of the 4th, 5th and 6th groups of the periodic table including the rare earths.

S. An electron discharge device comprising an electron gun adapted and arranged to generate and direct an electron beam along a predetermined beam axis, said gun being disposed at one end of said axis, a collector structure disposed at the other end of said axis, interaction means disposed along said axis between said gun and collector for providing an interaction means for said electron beam, and an ion draining electrode asymmetrically disposed within said collector structure with respect to the beam axis, said ion draining electrode being adapted and arranged to collect ions thereon, and a vacuum monitoring instrument operatively connected to said ion draining electrode.

6. An electron discharge device having an electron beam axis including a collector assembly disposed about said axis, said collector assembly comprising an ion draining electrode adapted and arranged to collect positive ions thereon and asymmetrically disposed in said collector assembly with respect to said axis, said ion draining electrode being partially constructed from a material selected from the group of reactive materials consisting of titanium, strontium, zirconium and any other transition elements of the 4th, 5th and 6th groups of the periodic table including the rare earths.

7. An electron discharge device having an electron beam axis including a collector assembly disposed about said axis, said collector assembly comprising an ion draining electrode adapted and arranged to collect positive ions thereon and asymmetrically disposed in said collector assembly with respect to said axis, said ion draining electrode having a portion thereof made of a material which upon bombardment by ions or electron particles will sputter and thereafter function as a gas absorbing substance to thereby vacuum pump said device.

8. An electron discharge device having an electron beam axis including a collect-or assembly disposed about said axis, said collector assembly comprising an ion draining electrode adapted and arranged to collect positive ions thereon and asymmetrically disposed in said collector assembly with respect to said axis, said ion draining elecsecondary electrons caused by electron beam impingement on the internal collector walls will be deflected away from the beam axis.

9. An electron discharge device comprising an elec tron gun adapted and arranged to generate and direct an. electron beam along a predetermined beam axis, said gun being disposed at one end of said axis, a collector structure disposed at the other end of said axis, interaction means disposed along said axis between said gun and. collector for interaction with said electron beam, and an ion draining electrode asymmetrically disposed within said collector structure with respect to the beam axis, said ion draining electrode being adapted and arranged to collect positive ions thereon, said ion draining electrode being biased with respect to said collector such that secondary electrons caused by electron beam impingement on the internal collector walls will be deflected away from the beam axis.

10. An electron discharge device having an electron beam axis including a collector assembly disposed about said axis, said collector assembly comprising an ion draining electrode adapted and arr-angedt-o collect ions thereon and asymmetrically disposed in said collector assembly with respect to said axis, said ion draining electrode being biased with respect to said collector such that secondary electrons caused by electron beam impingement on the internal collector walls will be deflected away from the beam axis, said ion draining electrode having a vacuum monitoring instrument operatively connected thereto.

11. An electron discharge device comprising an electron gun adapted and arranged to generate and direct an electron beam along a predetermined beam axis, said gun being disposed at one end of said axis, a collector structure disposed at the other end of said axis, interaction means disposed along said axis between said gun and collector for interaction with said electron beam, an ion draining electrode asymmetrically disposed within said collector structure with respect to the beam axis, said ion draining electrode being adapted and arranged to collect ions thereon, said ion draining electrode being biased with respect to said collector such that secondary electrons caused by electron beam impingement on the internal collector walls will be deflected away irorn the beam axis, said ion draining electrode having a vacuum monitoring instrument operatively connected thereto.

12. A multicavity klyst-ron oscillator device comprising electron gun means adapted and arranged to generate and direct an electron beam along a predetermined axis, said electron gun being disposed at one end of said predetermined axis, a pair of cavity resonators operatively connected to said electron gun means and disposed along said predetermined axis, collector means disposed at the'other end of said predetermined axis and operatively connected to said pair of cavity resonators, said collector means having an asymmetrically disposed ion draining electrode therein, said asymmetry being with respect to said predetermined axis, said ion draining electrode including means .for vacuum pumping said device.

References Cited by the Examiner UNITED STATES PATENTS 2,547, 00 4/ 1951 Dorgel-o 31364 2,996,639 8/ 1961 Jepsen 3155.38 3,213,314 Ill/ 1965 Reverdin 3137 DAVID J. GALVIN, Primary Examiner. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING AN ELECTRON GUN ADAPTED AND ARRANGED TO GENERATE AND DIRECT AN ELECTRON BEAM ALONG A PREDETERMINED BEAM AXIS, SAID GUN BEING DISPOSED AT ONE END OF SAID AXIS, A COLLECTOR STRUCTURE DISPOSED AT THE OTHER END OF SAID AXIS, INTERACTION MEANS DISPOSED ALONG SAID AXIS BETWEEN SAID GUN AND COLLECTOR FOR INTERACTION WITH SAID ELECTRON BEAM, AND AN ION DRAINING ELECTRODE ASYMMETRICALLY DISPOSED WITHIN SAID COLLECTOR STRUCTURE WITH RESPECT TO THE BEAM AXIS AND RADIALLY SPACED FROM SAID BEAM AXIS, SAID ION DRAINING ELECTRODE BEING ADAPTED AND ARRANGED TO COLLECT IONS THEREON. 